Bidirectional counting techniques are used to correct automatically for vibration or retraced motion ensuring that the fringe count represents the displacement of the moving reflector. For optimum performance bi-directional counters require two signals with constant average dc levels and sinusoidal components, related to the optical path difference in the interferometer, that are in phase quadrature. The counter logic is set to respond each time one of the signals passes through its average value. Unfortunately, in practice this dc level is subject to variations. For example, its value is dependent on the intensity of the light source. Removal of the dc component by capacitance coupling, so that the average signal level is always zero, is not completely effective because the frequency of the sinusoidal component may be very low and, indeed, zero if the corner cube retro-reflector attached to the workpiece is stationary.
It is common practice either to employ some form of modulation of the interferometer signal or to maintain the average signal level at zero volts by an electronic subtraction process which removes variations in the average signal level from the photodetector signals and avoids the need for modulation. However, instruments using these methods involve the use of polarization techniques, introducing a 90.degree. phase difference between the signals obtained from two orthogonally polarized components by means of a phase retardation plate. This results in the imposition of alignment requirements on the polarization azimuths of the optical components in the interferometer system and the radiation source and in addition increases the overall cost.